A general switching power source device is provided with an overcurrent protection circuit which limits its supply capacity so that there is no flow of an output current exceeding a predetermined value. The object of this is, as well as preventing an electronic instrument, or the like, from overheating or burning out due to an excessive current continuing to flow when the electronic instrument, or the like, which is a load connected to an output, breaks down due to low impedance, to prevent a breakdown, overheating, or burn out of the switching power source device itself.
Meanwhile, among the various kinds of electronic instrument which form the load of the switching power source device, in the case of an instrument including electromagnetic drive system parts such as a motor and relay, or an instrument in which, being an electronic circuit including digital elements such as a CPU, a DSP, and an FPGA, a current flows at a uniform timing, there are those in which, although not a breakdown of the instrument, a large current (hereafter, a peak current) flows for an extremely short time. In this kind of case, provided that it is for a short time, it is possible to safely supply this kind of peak current, even with a switching power source device configured of small, low-capacity power parts. Then, for example, an allowable, predetermined setting value (hereafter, an overcurrent setting value), which is a value exceeding a peak current value necessary for a normal operation of the load, being set as an upper limit value of the output current, there has been put into practical use a peak overcurrent protection circuit, or the like, which carries out an operation wherein it monitors a duration time of the peak current with a timer circuit configured of a capacitor, a resistor, and the like, and, in the event that the time exceeds a predetermined setting time, changes the overcurrent setting value to a small value.
Generally, it is often the case that the heretofore described kind of overcurrent protection circuit is an analog control type configured of discrete parts. Consequently, for example, in order to provide it with a function which adjusts its control characteristic to a control characteristic such as the overcurrent setting value in line with an operational condition of the electronic instrument, or the like, there is a need for complicated circuits which carry out various signal processes. Furthermore, it being unavoidable to configure the circuits with a large number of discrete parts, not only are an evaluation of the circuit operations and other aspects of management troublesome, but also, due to an increase in the number of parts, it is one cause of a hindering of a miniaturization and cost reduction of the switching power source device.
Meanwhile, an overcurrent protection circuit using a digital control can easily carry out an adjustment or switching of the control characteristics by embedding the complicated signal process in a computing program, and has a superior intelligence. For example, as disclosed in Patent Document 1, there has been proposed a switching power source device overcurrent protection circuit which includes a current detecting resistor, which converts an output current supplied to a load into a voltage signal, an analog/digital converter (hereafter called an A/D converter), which converts the voltage signal into a digital value, computing means which determines, based on the digital value, whether or not the output current is in an overcurrent condition exceeding an overcurrent setting value, and pulse width varying means which, when it is determined that there is an overcurrent condition, carries out a process of compulsorily shortening a pulse width of a switching element. The computing means and pulse width varying means are configured using a digital processor. The digital processor, being one which includes an addition and subtraction function, a multiplication and division function, a comparison function, a timer function, a storage function, and the like, can store various setting values or target values set from the exterior, using a storage function such as a memory or register.
Furthermore, in the heretofore described switching power source device, a first setting value, and a second setting value lower than the first setting value, being given as overcurrent setting values, the output current is limited based on the first setting value when first reaching the overcurrent condition, and limited based on the second setting value after a predetermined time elapses. Specifically, a comparison of the digital value of the output current, and the first setting value and second setting value, is computed by the comparison function of the digital processor, and when it is determined, as a result of the computation, that the digital value has exceeded the first setting value, the pulse width is compulsorily made narrower. Furthermore, a control is such that, even when the digital value has not exceeded the first setting value, when it is determined that a condition of exceeding the second setting value has continued for a predetermined time or longer, the pulse width is compulsorily made narrower.
Patent Document 1: JP-A-2001-119933